No storage tankFat has adipose tissue. Carbohydrate has glycogen. Protein has neither — every gram of body protein is already employed in one of the six jobs below. So when amino acids run short, the body does not spend a reserve; it borrows from working tissue, and something on this list gets less.
What the evidence supports
StrongProtein's six functional roles, the absence of a storage depot, and the yield of 4 kcal per gram are long-settled biochemistry. So is the finding that sustained protein-and-energy inadequacy impairs antibody production and wound healing.
PreliminaryWhether intakes above the RDA better preserve muscle in older adults, and whether spreading protein evenly across meals matters, are active research questions. Trials disagree, and no single food or supplement has been shown to prevent or treat a disease on its own.
Not individual advice. Everything here is population physiology and reference values used to plan and assess diets for groups. Requirements shift with age, pregnancy, lactation, activity, injury, and kidney or liver function. Nothing here diagnoses, treats, or prescribes — and no food here is "good" or "bad."